This is getting more interesting. Since you mentioned the pieces on the bottom were actually much larger than the one in your picture, THIS sheds a completely new light on things. I had previously considered the chunk of stuff might be "Clinkers"...which would come from burning coal. They are congealed impurities found in the ashes left in the firebox under the boiler. Like a wood fire, sometimes the coal did not completely burn..because it may have been low grade..ie..had impurities.
On page 134 of
"Farewell Old Mount Washington" there is a copy of an engineering drawing of the Old Mounts engine and boiler. There is an engineering note at the top of the drawing that describes the "Ash Disposal Unit"...as follows:
"Ash disposal unit: Ashes were shoveled into the opening and washed overboard by a jet of water through the 10" pipe and ash port, then ground up by the paddle wheel".
NB
http://en.wikipedia.org/wiki/Clinker_(waste)
EDIT: The Wiki link is Cranky. Click on "Did you Mean".....
Clinker (Waste)